We seek to quantitate the effects of transient blood-brain barrier disruption (BBBD) by mannitol in patients with malignant brain tumors undergoing chemotherapy. Sequential regional measurements of blood flow and unidirectional transport rate constants will be determined using positron emission tomography (PET) before and after transient barrier disruption. Patients will receive BBBD followed by chemotherapy in monthly intervals if they continue to meet the medical inclusion criteria. PET scanning will be done at the time of the first blood brain barrier disruption. Our goal is to determine whether the delivery of chemotherapeutic agents to tumoral and peri-tumoral regions can be enhanced by BBBD.